Bimetallic titania-based electrocatalysts deposited on ionic conductors for hydrodesulfurization reactions

ABSTRACT

This invention relates to a method for preparing a bimetallic titania-based catalyst for use in hydrodesulfurization reactions.

FIELD OF THE INVENTION

The present invention relates generally to the removal of sulfur fromhydrocarbon streams and, more particularly, to a catalytichydrodesulfurization process which allows for the in situ control ofcatalyst activity and selectivity.

BACKGROUND OF THE INVENTION

The passage of time has seen the enactment of ever more stringentregulations by governmental authorities based on the need to control andlimit sulfur emissions from vehicle exhaust. This requires the petroleumindustry to continually improve and upgrade their refinery processes todecrease the quantity of sulfur present in gasoline. Many countriesaround the world currently limit the allowable sulfur content to lessthan 50 ppm, and in some cases, as low as 20 ppm.

In many of the processes employed in the petroleum industry, hydrogen isreacted with organic hydrocarbon feedstocks in order to achieve certaindesired objectives. For example, in hydrocracking it is sought tomaximize the distillable fractions in oil, or its fractions. Inhydrodesulfurization (HDS), the aim is the reduction of the sulfurcontent.

In the aforementioned processes, hydrogen is reacted with thehydrocarbon in a chemical reactor containing a catalyst. The catalystenhances the process by increasing the reaction rate and also increasingthe selectivity of the desired reaction.

Catalytic desulfurization is a preferred method for the removal ofsulfur from hydrocarbons. Generally, catalytic desulfurization takesplace at elevated temperature and pressure in the presence of hydrogen.At the elevated temperatures and pressures, catalytic desulfurizationcan result in the hydrogenation of other compounds, such as for example,olefin compounds, which may be present in the petroleum fraction whichis being desulfurized. Hydrogenation of olefin products is undesirableas the olefins play an important role providing higher octane ratings(RON) of the feedstock. Thus, unintentional hydrogenation of olefincompounds during desulfurization may result in a decreased overalloctane rating for the feedstock. If there is significant loss of octanerating during the hydrodesulfurization of the hydrocarbon stream,because of saturation of olefin compounds, the octane loss must becompensated for by blending substantial amounts of reformate, isomerateand alkylate into the gasoline fuel. The blending of additionalcompounds to increase the octane rating is typically expensive and thusdetrimental to the overall economy of the refining process.

Additionally, catalytic hydrodesulfurization can result in the formationof hydrogen sulfide as a byproduct. Hydrogen sulfide produced in thismanner can recombine with species present in the hydrocarbon feed, andcreate additional or other sulfur containing species. Olefins are oneexemplary species prone to recombination with hydrogen sulfide togenerate organic sulfides and thiols. This reformation to produceorganic sulfides and thiols can limit the total attainable sulfurcontent which may be achieved by conventional catalytic desulfurization.

Alumina is a common support material used for catalyst compositions, buthas several disadvantages in the desulfurization of petroleumdistillates. Alumina, which is acidic, may not be well suited for thepreparation of desulfurization catalysts with high loading of activecatalytic species (i.e., greater than 10 weight %) for catalyticallycracked gasoline. Acidic sites present on the alumina support facilitatethe saturation of olefins, which in turn results in the loss of octanerating of gasoline. Additionally, recombination of the olefin withhydrogen sulfide, an inevitable result of hydrode sulfurization,produces organic sulfur compounds. Furthermore, basic species present inthe feedstock, such as many nitrogen containing compounds, can bind toacidic sites on the surface of the alumina and the catalyst, therebylimiting the number of surface sites which are available for sulfurcompounds for desulfurization. Furthermore, basic species present in thefeedstock, such as many nitrogen containing compounds, can bind toacidic sites on the surface of the alumina and the catalyst, therebylimiting the number of surface sites which are available for sulfurcompounds for desulfurization. At the same time, nitrogen containingcompounds having aromatic rings are easily transformed into cokeprecursors, resulting in rapid coking of the catalyst. Additionally,high dispersion of the metal is difficult to enhance with an aluminasupport due to the strong polarity and the limited surface area of thealumina. Exemplary commercially available hydrotreating catalystsemploying an alumina support include, but are not limited to,CoMo/AI₂0₃, NiMo/AI₂0₃, CoMoP/AI₂0₃, NiMoP/AI₂0₃, CoMoB/AI₂0₃,NiMoB/AI₂0₃, CoMoPBI Al₂0₃, NiMoPB/AI₂0₃, NiCoMo/AI₂0₃, NiCoMoP/AI₂0₃,NiCoMOB/AI₂0₃, and NiCoMoPB/AI₂0₃, (wherein Co is the element cobalt, Niis nickel, Mo is molybdenum, P is phosphorous, B is boron and Al isaluminum).

In addition, prior art methods suffer in that the preparation ofdesulfurization catalysts having high metal loading with high dispersionis generally difficult. For example, many prior art catalysts areprepared by a conventional impregnation method wherein the catalysts areprepared by mixing the support materials with a solution that includesmetal compounds, followed by filtration, drying, calcination andactivation. However, catalyst particles prepared by this method aregenerally limited in the amount of metal which can be loaded to thesupport material with high dispersion, which generally does not exceedapproximately 25% by weight of the metal oxide to the support material.Attempts to achieve higher loading of the metal to support materialshaving a relatively high surface area, such as silicon dioxide,typically result in the formation of aggregates of metallic compounds onthe surface of the support. Activated carbon has much higher surfacearea and weaker polarity than conventional catalyst supports, such asfor example, alumina and silica. This provides improved performance inthe desulfurization of catalytically cracked gasoline because botholefin saturation and recombination of hydrogen sulfide with the olefinare suppressed over activated carbon support. However, weaker polarityand a relatively high hydrophobicity make activated carbon difficult toload large amount of active metallic species, such as molybdenum oxide.

It can be seen from the foregoing that methods for enhancing theperformance of catalysts useful in the removal of sulfur species frompetroleum-based products are needed.

SUMMARY OF THE INVENTION

The present invention provides an electrochemical catalytic method forthe hydrodesulfurization of a petroleum-based hydrocarbon stream whichcomprises contacting the petroleum-based hydrocarbon stream with ahydrogen-containing gas in an electrochemical call employing Non FaradicElectrochemical Modification of Chemical Activity, said cell comprisingan active metal catalyst working electrode applied to a chargeconducting solid electrolyte, which is connected to a counter electrode,and is electrochemically promoted by applying a current or potentialbetween the catalyst and the counter electrode duringhydrodesulfurization.

The present invention also, provides a method for the preparation of abimetallic titania-based catalyst for use in hydrodesulfurizationreaction, which comprises

a) dissolving a salt of a Group VI metal of the Periodic Table in waterand adjusting the pH of the solution to an acidic value;

b) dissolving a titanic compound in the solution of step a) andadjusting the pH of the solution to an acidic value;

c) dissolving a salt of Group VIII metal of the Periodic Table in thesolution of step b) and adjusting the pH of the solution to an acidicvalue;

d) evaporating the solution of step c) at elevated temperature andpressure and collecting a bi-metallic, titania-based solid; and,

e) calcining the bi-metallic, titania-based solid at an elevatedtemperature.

It has been found that in the hydroprocessing of hydrocarbon streams,the reaction rate can be enhanced, beyond the normal catalyticenhancement, by applying an electrical potential or current to thesurface of the catalyst. By applying this electrical potential, theelectron density on the surface of the catalyst is changed, whichresults in promoting or increasing the hydroprocessing reaction rate,e.g., the hydrodesulfurization (HDS) rate.

The application of an electrical potential to the surface of a catalystis referred to as the NEMCA effect (Non-Faradic ElectrochemicalModification of Catalytic Activity). The NEMCA effect is a phenomenonwherein the application of small currents and voltage potentials oncatalysts in contact with solid electrolytes leads to pronounced,strongly non-Faradic and reversible changes in both catalytic activityand selectivity.

In the hydroprocessing of hydrocarbon streams, particularlyhydrodesulfurization, in accordance with the present invention, theNEMCA effect is applied to good advantage. The effect is based on thediscovery that by applying an electric voltage between, on the one hand,an active material which is applied, preferably in the form of layers,to a solid electrolyte and, on the other hand, a further metallicsubstrate, also preferably in the form of layers, which is in turnconnected to a solid electrolyte, it is possible to increase theactivity (rate) and selectivity of a catalyst.

Electrochemical promotion allows for in situ control of catalystactivity and selectivity by controlling in situ the promoter coveragevia potential application.

In traditional catalytic processes, classical promoters are used, whichtypically are added during catalyst preparation, to activate a catalyticprocess. Another option is the use of metal-support interactions, whichactivates the catalytic function by using an active support. Neither ofthese approaches, however, provides accurate and on-demand dosage ofpromoters during reaction conditions.

The use of NEMCA technology allows for the precise dosing ofelectropromoters to a catalyst surface during reaction conditions byadjusting the flux of ions (promoters) to the catalyst surface bycontrolling the applied current or voltage to the cell.

Thus, in one embodiment, a method for the hydrodesulfurization of apetroleum based hydrocarbon distillate of crude oil is provided thatincludes the step of contacting the petroleum hydrocarbon distillatewith hydrogen gas in the presence of a catalyst which has beenelectrochemically enhanced by the NEMCA effect.

In another embodiment, a hydrodesulfurization catalyst composition isprovided whose rate of activity is enhanced by the NEMCA effect.

In still another embodiment, a method is provided for the preparation ofa bi-metallic hydrodesulfurization catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the transient effect of a constant applied negativepotential (−1.5V) on the rate of H₂S formation, the conversion ofthiophene and the current. T=250° C., sample: S1.

FIG. 2 depicts the transient effect of a constant applied positive(1.5V) and negative (−1.2V) potential on the rate of H₂S formation, theconversion of thiophene and the current. T=500° C., sample: S2.

FIG. 3 depicts the transient effect of a constant applied negativecurrent (−1 μA) on the H₂S formation catalytic rate, the conversion ofthiophene and the catalyst reference potential difference. T=300° C.,sample: S3.

DETAILED DESCRIPTION OF THE INVENTION

The NEMCA effect on hydrodesulfurization catalysts can best be describedas an electrochemically induced and controlled promotion effect ofcatalytic surfaces generated by electrolyte charge carrier spilloverto/from the electrolyte onto the catalyst surface.

In one embodiment of the present invention, a catalyst composition isprovided for the removal of sulfur from petroleum hydrocarbon oils. Thecatalyst composition is useful in the removal of sulfur from middledistillates produced at distillation temperatures ranging from 200° C.to about 450° C., for example diesel fuel.

In the hydrodesulfurization (HDS) process of the present invention, thecatalyst can consist of at least one metal from the metals of Group VIIIof the Periodic Table and at least one metal selected from the metals ofGroup VI of the Periodic Table, which are used as the active metals tobe supported on the support.

Examples of the Group VIII metals include cobalt (Co) and nickel (Ni),while examples of the Group VIA metals include molybdenum (Mo) andtungsten (W). The combination of the Group VIII metal and the Group VImetal is preferably Mo—Co, Ni—Mo, Co—W, Ni—W, Co—Ni—Mo, or Co—Ni—W, andmost preferably Mo—Co or Ni—Mo.

The content of the Group VI metal in terms of its oxide is preferably inthe range of about 1% to 30% by mass, more preferably 3% to 25%, bymass, and most preferably 5% to 20% by mass, based on the mass of thecatalyst. If less than 20% by mass is employed, it would not besufficiently active to desulfurize sufficiently, and if a mass greaterthan 30% were employed, it would condense resulting in reduceddesulfuzation

The supporting ratio of the Group VIII metal and the Group VI metal is amolar ratio defined by [Group VIII metal oxide]/[Group VI metal oxide]ranging from 0.105 to 0.265, preferably 0.125 to 0.25, and mostpreferably from 0.15 to 0.23. A molar ratio of less than 0.105 wouldresult in a catalyst having inadequate desulfurization activity. A molarratio of greater than 0.265 would result in a catalyst lackingsufficient hydrogenation activity and reduced desulfurization activity.

The total content of the Group VIII metal and the Group VI metal ispreferably 22% by mass or greater, more preferably 23% by mass orgreater, and most preferably 25% by mass or greater in terms of oxidebased on the mass of the catalyst. A mass of less than 22% would resultin a catalyst which exerts insufficient desulfurization activity.

The preferred catalyst support for use in accordance with the presentinvention is titanium dioxide (TiO₂). While alumina is the most widelyused support material for commercial hydrodesulfurization (HDS)catalysts due to its good mechanical properties, titania based catalystshave been found to be more successful and more suitable when the HDSprocess is based on electrochemical promotion.

Other supports can also be employed provided they are ion conductors.Exemplary of such supports are alumina, ceria, silica, zirconia, RuO₂,CZl and BCN18.

The solid electrolyte supports which can be utilized in the process ofthe present invention are O²⁻ ionic conductors, exemplary of which isYSZ (8% mol Yttria Stabilized Zirconia) and low temperature (<400° C.)proton conductors, exemplary of which is BCN 18 (Ba₃ CA_(1.18)Nb_(1.82)O_(9-a)).

Other solid electrolytes include β¹¹-Al₂O₃, β-Al₂O₃, Li⁺ and K⁺conducting β-Al₂O₃.

The level of the voltage applied is usually in the range of ±0.5V to±2V, preferably about ±1.5V.

As adverted to previously, the phenomenon of electrochemical promotionof catalysis (EPOC or NEMCA effect) has been utilized in the process ofthe present invention for the in situ modification of the HDS activityof bimetallic Mo—Co catalyst-electrodes at low temperatures andatmospheric pressure.

The phenomenon of electrochemical promotion of catalysis has beeninvestigated using a variety of metal catalysts (or conductive metaloxides), solid electrolyte supports and catalytic reactions. Inelectrochemical promotion, the conductive catalyst-electrode is incontact with an ionic conductor and the catalyst is electrochemicallypromoted by applying a current or potential between the catalyst filmand a counter or reference electrode, respectively. Numerous surfacescience and electrochemical techniques have shown that EPOC is due toelectrochemically controlled migration (reverse spillover orbackspillover) of promoting or poisoning ionic species (O²⁻ in the caseof YSZ, TiO₂; and CeO₂, Na⁺ or K⁺ in the case of β″-Al₂O₃, protons inthe case of Nafion, CZI (CaZr_(0.9)In_(0.1)O_(3-α)) and BCN18(Ba₃Ca_(1.18)Nb_(1.82)O_(9-α)), etc.) between the ionic or mixedionic-electronic conductor—support and the gas exposed catalyst surface,through the catalyst-gas-electrolyte three phase boundaries (TPBs).

Two parameters are commonly used to quantify the magnitude of the EPOCeffect:

1. the rate enhancement ratio, ρ, defined from:ρ=r/r _(o)  (1)

where r is the electropromoted catalytic rate and r_(o) theopen-circuit, i.e. normal catalytic rate.

2. the apparent Faradaic efficiency, Λ, defined from:Λ=(r−r _(o))/(I/nF)  (2)

where n is the charge of the ionic species and F the Faraday's constant.

A reaction exhibits electrochemical promotion when |Λ|>1, whileelectrocatalysis is limited to |Λ|≦1.

The selectivity of the reaction to the produced hydrocarbon (HC) specieshas been calculated byS _(i) =r _(i) /r _(th)  (3)

where, r_(i) is the formation rate of each HC product and r_(th) theconsumption rate of thiophene, equal to Σr_(i) within ±2%.

The preferred method for preparing MoCo/T₁O₂ for use in accordance withthe present invention involves wet impregnation at various pH values.The wet impregnation method was used for the co-deposition of Mo and Cowith a ratio of 15 wt % MoO₃ to 3 wt % CoO. The addition of 82 wt % TiO₂(anatase) ensures a fine dispersion of the catalyst with only onemonolayer of mostly Mo at the surface of the TiO₂ matrix. As indicated,wet impregnation can be carried out at different pH values. At moreacidic concentrations of the solution (pH=4 or 4.3) the formation ofpolymeric Mo₇O₂₄ is to be expected, while at neutral conditions(pH=6.7), only monomeric MoO₄ species will be obtained.

The polymeric species (mainly [Mo₇O₂₄]⁶⁻ and [HMo₇O₂₄]⁵⁻) are depositedthrough electrostatic adsorption on TiO₂ surface. The monomeric species(mainly [MoO₄]²⁻) are adhered through the formation of hydrogen bondsand inner sphere complexes. Because the wet impregnation method is usedto prepare the catalysts, only a small amount of the species isdeposited through the above deposition modes. Thus, the greater amountof the molybdenum species is deposited through bulk deposition. Thismeans that the species are deposited through precipitation during thewater evaporation step.

Preparative Example 1

Wet Impregnation Method at pH 4.3

The preparation of the CoMo/TiO₂ (pH=4.3) catalyst is performed usingthe “co-impregnation under EDF conditions” method. According to thismethod, 0.7361 g ammonium heptamolybdate [(NH₄)₆Mo7O24*4H₂O] weredissolved in about 100 ml of triple-distilled water, in a 500 ml roundflask. The pH was adjusted to 4.3 by adding concentrated HNO₃ solutiondropwise. To this solution, 3.495 g of TiO2 were added and due to the Moadsorption on TiO₂ surface and the PZC(=6.5) of the titania used, the pHrose to between 8 and 8.5. To this suspension, 0.52018 g of cobaltnitrate [Co(NO₃)2*6H₂O] were then added. Before the addition of thecobalt nitrate salt the pH was adjusted again to 4.3 in order to avoidCo(NO₃)OH precipitation. As the adsorption of cobalt species altered thesolution pH to a value of 3.0-3.5, the latter had to be adjusted againto 4.3 using a concentrated ammonia solution (NH₄OH).

The round flask containing the suspension was then placed in a rotaryevaporator and stirred for about 30 min. After a final adjustment to pH4.3, the evaporation at T=40-45° C. and pressure 30 mbar was started.When the material was dried, it was transferred in a porcelain crucibleand calcined at 500° C. for 2 hours in air. The final composition of thecatalyst was 14.2 (10) wt % MoO₃ (Mo), 3.2 (2.6) wt % Coo (Co) and 82.6(87.4) wt % TiO₂.

Preparative Example 2

Co-Deposited Catalyst Powder

A few drops of triple-distilled water was added to a small amount of thefinal powder of the wet impregnation method of Preparative Example 1 toform a thick paste, which was then spread at the surface of the protonion conductor. The catalyst film was dried at 120° C. for 30 minutes andwas then calcined at 500° C. for 2 hours.

Preparative Example 3

Procedure for Preparing Sputtered Mo Films

Thin Molybdenum (Mo) coatings are produced by a de magnetron sputterprocess. By this process homogeneous, well adhered, thin metal or metaloxide coatings are produced. For the production of such coatings,suitable for electrochemical promotion, several favorable depositionparameters need to be defined.

Deposition Parameter for the Mo Catalyst-Electrodes

The sputtering parameters for the Mo coating on the CIZ electrolytesubstrate with a mass of

m=0.0001 g are:

p=8.86-10.53 mTorr (˜40 ccm Ar)

P=330-403 Watt

I=0.8-1.09 Amp

V=327 Volt

Deposition Time: 10 minutes

The sputtering parameters for the Mo coating on the CIZ electrolytesubstrate with a mass of

m=0.0135 g are:

p=5.57-6.74 mTorr

P-306-390 Watt

I=0.8-1.03 Amp

V=340 Volt

Deposition Time: 40 minutes

Preparative Example 4

Procedure for Preparation of MoCo Deposited on Sputtered Mo Films

A thin layer of Mo was sputtered on the CZI electrolyte and calcined at500° C. The films were reduced in H₂ and presulfation was carried out at500° C.

The MoCo deposited catalyst was prepared in the following way:

1. To a round bottom flask, 100 ml of triple distilled water was added.

2. 3 g of (NH₄)₆Mo₇O₂₄*4H₂O was added to the 100 ml H₂O.

Preparative Example 5

Presulfation

Presulfation of the catalyst-electrolyte assembly was carried out withall catalyst-electrolyte assemblies in Examples I and II in thefollowing manner.

-   -   A stream of Ar (20 ml/min) was passed through the reactor,        discussed in Preparative Example 8, while the reactor was heated        to 500° C. and was maintained for 30 min at 500° C.    -   A stream of 15 Vol % H₂S and 85 Vol % H₂ was supplied to the        reactor at 500° C. for 2 hours.    -   After completing the presulfation, a stream of Ar was supplied        for 1 hour at 400° C.    -   Cooling/heating to the desired reaction temperature.

Preparative Example 6

Two Different Solid Electrolyte Supports were Used:

a. YSZ (8% mol Yttria Stabilized Zirconia), an O²⁻ ionic conductor.

b. BCN18 (Ba₃Ca_(1.18)Nb_(1.82)O_(9-α)) a low temperature (<400° C.)proton conductor.

Both of the solid electrolyte pellets had a thickness of 2 mm anddiameter of 18 mm.

Preparative Example 7

Preparation of Metal and Metal Oxide Interlayers

Thin Mo and TiO₂ layers were deposited over the ionic conductorsupports, before the catalyst deposition, to increase the intimatecontact or adhesion between the catalyst-electrode, also known as theworking electrode, and the support, and to facilitate the migration ofthe backspillover species onto the catalyst surface. The enhancing roleof TiO₂ interlayers has been reported in previous EPOC studies, both foroxidation and hydrogenation reactions.

The Mo/electrolyte and TiO₂/electrolyte type thin electrodes wereprepared by metal sputtering as described previously. A magnetronsputtering system was used. High purity Ar and O₂ were used assputtering and reactive gas, respectively. The discharge characteristicswere controlled using a variable DC power supply (1 kV and 2 A). Pure Mo(99.95%) and Ti (99.95%) were used as sputtering targets.

TiO₂ thin layers were deposited only over the YSZ substrates with 600 Wtarget power, which enabled a 0.5 nm/min deposition rate and led to ac.a. 90 nm thickness film after 3 hours of deposition. The substratetemperature was stable during the deposition at 250° C. Also, apost-deposition annealing of the deposited TiO₂ layer was performed inair at 600° C. for 60 minutes, resulting in a 60% rutile and 40% anatasestructure. The TiO₂ layers structure characterization has been describedpreviously.

Molybdenum interlayers were deposited in each case holding the targetpower stable at 280 W, which enabled a 20 nm/min deposition rate,achieving a 100 nm thickness Mo layer after 5 minutes of deposition. Thesubstrate temperature was kept stable at 50° C.

On the other side of the pellets, Au counter and reference electrodeswere prepared in each case by application of a metalorganic paste(Metalor, Gold resinate, A1118), followed by drying at 400° C. for 90minutes and calcination at 650° C. for 30 minutes. Blank experimentsusing Au also for the working electrode showed that Au is practicallycatalytically inactive for the HDS reaction, establishing that theobserved rate values correspond only to the rate on thecatalyst-electrode and not to the counter or reference electrodes.Similar blank experiments with TiO₂ powder showed that TiO₂ also isinactive under the mentioned conditions.

Preparative Example 8

Reactor Operation

An atmospheric pressure electrochemically promoted single chamberreactor was used equipped with a solid electrolyte pellet, on whichthree electrodes were deposited, namely, the working-catalyst electrode,the counter electrode and the reference electrode. Gold wires were usedfor the electrical connections between the electro catalytic element andthe external power supply unit. A three-bore, ultra high vacuumfeed-through unit, special for high temperature H₂ and H₂S environment,was used for the gastight introduction of the gold wires in the reactor.The temperature was measured and controlled by a type-K thermocoupleplaced in a stainless steel close-end tube in the proximity of thecatalyst electrode.

The feed gas composition and total gas flow rate, F_(T), was controlledby four mass flowmeters (Brooks smart mass flow and controller, B5878).Reactants were Messer-Griesheim certified standards of pure (99.99%) H₂and H₂S, while thiophene was introduced using a saturator (P_(th)^(25° C.)=3 kPa) with Ar or H₂ carrier gas. Pure (99.99%) argon was fedthrough the fourth flowmeter in order to further adjust total gas flowrate and inlet gas composition at desired levels. H₂ partial pressurewas held constant at 97 kPa, while thiophene partial pressure could bevaried from 0.5 to 5 kPa. The total gas flow rate was constant at 30 and60 cm³/min. Reactants and products were analyzed by on-line gaschromatography (Shimadzu 10A, equipped with a Porapaq-QS column at 50°C. for the separation of thiophene and the produced C_(x)H_(y)) inconjunction with a continuous analysis H₂S colorimeter (AppliedAnalytics Inc.). Constant currents or potentials were applied using anAMEL 2053 galvanostat-potentiostat.

TABLE 1 Electrochemical Cells Used in Examples Counter/ WorkingReference Sample Cell Electrode Interlayer Electrolyte Electrode S1MoCo/TiO₂/YSZ/Au MoCo TiO₂ YSZ Au (impregnation) (sputtering) S2MoCo—TiO₂/ MoCo/TiO₂ Mo YSZ Au Mo/YSZ/Au (dispersed- (sputtering)impregnation) S3 MoCo—TiO₂/ MoCo/TiO₂ Mo BCN18 Au Mo/BCN18/Au(dispersed- (sputtering) impregnation)

Example I. The Use of O²⁻ Ionic Conductor Support (YSZ)

As can be seen by an examination of FIG. 1, it shows the transienteffect of a constant applied negative potential (−1.5V) on the rate ofH₂S formation (reaction (4)), the conversion of thiophene and thecurrent at 250° C., using sample S1, i.e. Mo—Co/TiO₂(sp)/YSZ/Au,identified more fully in Table 1 and the preparative examples.C₄H₄S+H₂→C_(x)H_(y)+H₂S  (4)

As shown, under open circuit, i.e. normal catalytic conditions, theconversion of thiophene is ˜0.5%. Negative potential application (−1.5V)causes a 3.9-fold increase of the catalytic rate (ρ=3.9), wherethiophene conversion reaches ˜2%, while the apparent Faradaic efficiencyis 0.2. However, in the present instance where oxygen is not present inthe gas mixture, any current or potential-induced catalytic rate changesuggests electrochemical promotion, even when |Λ|<1. On the other hand,positive potential application has no effect on the catalytic rate. Theelectrophilic behavior observed here, i.e. rate increase by electrodepotential or work function decrease, appears when the electron acceptor(i.e. thiophene) species is weakly adsorbed on the catalytic surface andthe electron donor (H₂) strongly adsorbed. This agrees with the positiveorder dependence of the reaction rate on P_(th), proposed in theliterature. Upon the application of negative potential, i.e. O²⁻ pumpingfrom the catalytic electrode, the thiophene-catalyst bond strengthincreases and causes higher thiophene coverage of the catalytic surface.

After current interruption the catalytic rate reversibly returns to itsinitial open-circuit steady-state value. This indicates that the surfacesulfur species, formed during the sulfation pretreatment step, were notconsumed upon negative polarization, which would cause catalyst partialdeactivation.

As can be seen by an examination of FIG. 2, it shows the transienteffect of a constant applied positive (1.5V) and negative (−1.2V)potential on the rate of H₂S formation (reaction (4) above), theconversion of thiophene and the current at 500° C., using sample S2identified more fully in Table 1 and the preparative examples, where theCoMo—TiO₂ dispersed catalyst coating is supported over a thin sputterdeposited Mo film interfaced with YSZ.

As shown, under open-circuit conditions the conversion of thiophene is˜0.7%, similar to that obtained using sample S1 (FIG. 1) at 250° C.Positive potential application (1.5V) causes an 1.5-fold increase of thecatalytic rate, while the apparent Faradaic efficiency is 0.1. This isin contrast to the behavior of sample S1, where positive polarizationhad no effect on the rate. On the other hand, application of a negativepotential (−1.2V) causes a similar less pronounced effect on the ratewhere ρ=1.5, indicating the inverted volcano behaviour of the system.This change from electrophilic to inverted volcano behavior, utilizingsimilar samples (S1 and S2), S2 is mainly due to the high operatingtemperature for sample S2. At elevated temperatures both electronacceptor and electron donor species are weakly bonded on the catalyticsurface, which results in the observed inverted volcano behavior of FIG.2. Similar change of the EPOC behaviour by temperature increase has beenreported in previous studies on the model reaction of C₂H₄ oxidationover Pt/YSZ, where the behaviour changes from electrophobic to invertedvolcano.

After positive current interruption, the catalytic rate decreases to avalue lower (?) than the initial open-circuit steady-state value,indicating a deactivation of the catalyst.

Example II. The Use of Proton Conductor Support (BCN18)

As can be seen by an examination of FIG. 3, it shows the transienteffect of a constant applied negative current (−1 μA) on the H₂Sformation catalytic rate, the conversion of thiophene and the catalystelectrode—reference potential difference at 300° C. using sample S3,identified more fully in Table 1 and the preparative examples.

As shown, negative current application causes a 10% increase of thecatalytic rate (ρ=1.1), while the apparent Faradaic efficiency equals585, i.e. each proton present on the catalyst surface, which establishesan effective double layer, causes the reaction of up to 585 adsorbedfrom the gas phase hydrogen species. After the negative currentinterruption, the catalytic rate decreases and stabilizes at a valuelower than the initial value. This “poisoning” effect of the negativepolarization can be attributed to possible hydrogenation of thecatalytically active surface sulfur groups supplied by the backspilloverproton species upon negative polarization.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and theelectrochemical cell disclosed herein may be made without departing fromthe scope of the invention, which is defined in the appended claims.

What is claimed is:
 1. A method of preparing a CoMo/TiO₂ catalyst usingco-impregnation under Equilibrium Deposition Filtration (EDF), whichcomprises the sequential steps of: a) first dissolving a molybdenum saltin water and adjusting the pH of the solution to an acid value; b)followed by dissolving a titania compound in the solution of step a) andadjusting the pH of the solution to an acid value; c) followed bydissolving a cobalt salt in the solution of step b) to adsorb saidcobalt onto said titania compound with said molybdenum and adjusting thepH of the solution to an acid value; d) evaporating the solution of stepc); e) collecting a CoMo/TiO₂ solid; and f) calcining the CoMo/TiO₂ toform a catalyst.
 2. The method according to claim 1, wherein the pH instep a) is adjusted to 4.3.
 3. The method according to claim 2, whereinthe pH of the solution in step a) is adjusted with HNO₃.
 4. The methodaccording to claim 1, wherein the pH in step b) is adjusted to 4.3. 5.The method according to claim 4, wherein the pH of the solution in stepb) is adjusted to 4.3.
 6. The method according to claim 1, wherein theevaporation in step e) is conducted at a temperature of about 40° C. toabout 45° C. and at a pressure of 30 mbar.
 7. The method according toclaim 1, wherein the calcinations in step e) is conducted at about 500°C. for about 2 hours.
 8. The method according to claim 1, wherein thefinal composition of the catalyst is about 14.2 wt. % MoO₃, 3.2 wt. %CoO and 82.6 wt. % TiO₂.
 9. The method of claim 1, wherein the acidvalue of (a), (b), and (c) is 4.3.
 10. The method of claim 1, whereinsaid Mo salt is ammonium heptamolybdate, said titania compound is TiO₂and said cobalt salt is cobalt nitrate.